The electrodes used are of paramount importance to the efficiency of the electrolysis process, i.e., in chlor-alkali or water electrolysis for obtaining caustic soda solution, chlorine, hydrogen or hydrogen peroxide. These are to have a resistance which is as low as possible, thereby permitting a rapid discharge and supply of the reaction gas from the reaction region between cathode and anode.
The conventional constructional electrode types in chlor-alkali or water electrolyzers with highly active electrode materials are still very inefficient due to gas bubble charging of the electrolyte and gas bubble covering of the electrodes in the active region.
So far the use of gas diffusion electrodes has most of the time not been possible in electrochemical processes because of the excessive operating costs and the unsatisfactory material exchange of the gas-enriched porous electrodes that is observed in liquid-filled cells. The relatively low strength of the electrode/diaphragm/electrode combination and the partly inhomogeneous current distribution have so far limited technical implementations to small units. The manufacture and operational use of very thin and stable electrodes requires expensive technologies.
French Patent Specification 2 308 701 discloses an electrolysis apparatus with refractionated electrodes wherein the porous electrodes are partly coated with porous metal oxides and it is ensured by way of a pressure drop that gas bubbles can only espace from the back side of the electrode. Both the pores of the electrode base body and the metal oxide coating are filled with electrolyte. Such electrodes are expensive and it is difficult to remove residual gases from the pores after the electrolytic cell has been switched off.
As for the production of chlorine and caustic soda solution, a cell design became recently known as a falling film apparatus wherein a very thin, oxygen-consuming porous cathode of Ag/Polytetrafluoroethylene PTFE is used (Chem.-Ing.-Tech. 60 (1988) No. 7, p. 563). Oxygen is supplied to the porous electrode on the one hand while diluted caustic solution passes, on the other hand, between said electrode and the membrane from above and leaves the cell at the bottom side as an enriched caustic soda solution. The anode of actively coated expanded titanium material of a conventional type, as well as the cathode having a gap of a width of, for instance, about 0.6 mm are arranged relative to the membrane and are flooded with brine from above. The resultant chlorine exists at the back side from the anode and is discharged through an adjoining liquid-free compartment.
Moreover, WO 91/00379 discloses an electrolytic cell for gas-developing electrolytic processes in which the problems entailed by gas discharge from the active region and by high electric load are to be solved by using at least one capillary gap electrode in that there is a motional direction of the gas bubbles through the electrolyte-filled electrode substantially in a direction perpendicular to the membrane or in the direction of the electric field between anode and cathode by way of a suitable dimension of the capillary gap. In this case, too, the above-mentioned difficulties are, however, not overcome in a satisfactory way.